A large variety of known sensors measure physical quantities based on a change of an electrical impedance. The electrical impedance “Z” consists of the real part “R” referred to as resistance and the imaginary part “X” referred to as reactance.Z=R+jX  (1)The real part “R” relates to electrical resistors and the imaginary part “X” may relate to capacitance and/or inductance values based on the well known terms ZC=1/jωC or ZL=jωL.
In order to determine the value of an unknown impedance, conventional systems use a known reference, as for example, a reference impedance, a reference voltage, a reference current, or time or other reference signals. However, not only the sought physical quantity may change the variable impedance. The precision of a system for impedance measurement can be adversely affected by changing operating conditions (i.e., temperature variations, supply voltage variations, humidity, and so forth). Therefore, known and unknown impedances are configured as a complex voltage divider in order to prevent or reduce negative effects. As the value of the imaginary part of impedance is frequency dependent, a time constant or resonant circuit can be formed for analyzing the frequency as a representation of the change in impedance. In this case, reference components can be included as well. There are various different implementations for this kind of devices. All of them are serving different purposes dedicated to the accuracy, speed and sensor dynamic range. Furthermore, in some cases, a fast reaction to system failure may be needed.
There are many different systems for measuring capacitance or inductance values. Some methods are accurate, some are fast and other low cost. Dependent on the individual requirements, different principles are applied. Capacitance value measurement is used in the industry but not as easy as measuring voltage, current or resistance. This is similar for measuring inductance values. The known principles are all based on generating an AC signal, either for excitation or as time constant or resonance frequency. However, the reaction time of the conventional systems is still too long and the precision too low.